1. Field of the Invention
The present invention relates to an improvement on a focus detecting device provided in an optical apparatus such as a camera.
2. Description of Related Art
Many of the single-lens reflex cameras of today are equipped with an automatic focusing device of the phase-difference detecting type using a line sensor. During recent years, the method of detecting a phase difference has variously been developed into a so-called area type automatic focusing method (hereinafter referred to as the area AF method) whereby focus detection is possible for any position within a two-dimensional focus detecting range.
FIG. 10 is a central sectional view of a mirror box of a camera, showing a focus detecting optical system designed for attaining the above-stated area AF method. In FIG. 10, reference numeral 101 denotes a lens mount. An objective lens (now shown) is mounted on the left side of the lens mount 101. A main mirror 103 is secured to a main-mirror receiving plate 114. That part of the main mirror 103 which is located on the optical axis 102 of the objective lens is formed as a semi-transparent area. A sub-mirror 104 is secured to a sub-mirror receiving plate 115 and is arranged on the optical axis 102 of the objective lens. Reference numeral 106 denotes a paraxial image forming plane, which is made conjugate to an image forming plane 105 by the sub-mirror 104.
Parts mounted on an AF body block 116, which is a base body of a focus detecting device, are next described as follows. A first AF mirror 107 which is a flat mirror is arranged to reflect a light flux coming from the sub-mirror 104. A mask member 108 is arranged to prevent the first AF mirror 107 from reflecting unnecessary rays of light. An infrared ray cutting filter 109 is arranged to remove such infrared rays that lower the accuracy of focus detection. A stop plate 110 is composed of a light-blocking thin plate which is made of a metal or resin and is provided with at least a pair of stop apertures formed therein. A reimaging lens block 111 includes at least a pair of lens parts corresponding to the stop apertures provided in the stop plate 110. A second AF mirror 112 is arranged to rectangularly reflect light fluxes coming from the reimaging lens block 111. An AF sensor 113 is a photoelectric conversion element having light receiving parts on which at least a pair of two-dimensional object images are projected by the reimaging lens block 111. An AF flexible circuit board 117 has the AF sensor 113 and its peripheral circuits (not shown) mounted thereon and is connected to a main circuit board (not shown). Further, a unit which is composed mainly of the above-stated AF body block 116 will be hereinafter simply called a focus detecting unit.
In a case where the area AF method is to be realized in accordance with the phase difference detecting method by the above-stated arrangement, however, the AF sensor tends to become too large in size, because the secondary object images which are projected in pair two-dimensionally spread. Therefore, to solve this problem, the above-stated optical system is arranged in a Z shape to make a ratio between the optical path length from the image forming plane 106 to the AF sensor 113 and the distance from the reimaging lens block 111 to the area sensor 113 as large as possible in such a way as to make the magnifying rate of reformed images on the AF sensor 113 as small as possible. By this arrangement, the AF sensor 113 can be arranged to be small enough for incorporation in the camera. Therefore, in order to keep the distance measuring accuracy of the focus detecting device of the above-stated area AF type at the same degree of accuracy as the conventional type focus detecting device, the AF sensor 113 must be arranged to be composed of minute pixels (picture elements). However, with the AF sensor 113 arranged to be composed of such minute pixels, focus detection would be greatly affected even a slight change in position of the secondary object images formed on the AF sensor 113.
One of inevitable factors of changes in position of the secondary object images is a change in distance between lenses caused by the thermal expansion of the reimaging lens block 111. Therefore, it has been developed and practiced to detect the temperature within the camera, particularly around the focus detecting unit by a temperature detecting means such as a temperature sensing IC or the like and to perform a temperature correcting action on the correlation of the secondary object images detected by the AF sensor 113.
However, since the temperature sensor which is the above-stated temperature detecting means sometimes has been used also for some purpose other than focus detection and thus has been disposed at a part other than the focus detecting unit. In such a case, it has been hardly possible to accurately make temperature correction as the temperature of the reimaging lens part of the focus detecting device differs from temperature measured by the temperature sensor. It has been also developed to have the temperature sensor disposed within the AF sensor. In that case, however, heat generation by the AF sensor itself tends to hinder accurate measurement of temperature of the reimaging lens part.